More than 5% of the population of the Western world above the age of 60 is affected by dementia. Two thirds of these cases are due to Alzheimer's disease (AD) (1-3). The prevalence of AD doubles every 5 years after age 65. Thus in persons older than 90 years of age there is a prevalence of more than 25% (3). The diagnosis of AD is confirmed by 2 major histopathologic hallmarks: senile plaques, which are extracellular deposits of amyloid β (Aβ) peptide, and neurofibrillary tangles, which are somatic inclusions of the microtubule-associated protein tau.
The Amyloid Cascade Hypothesis: Structure and Metabolic Processing of the Amyloid β Precursor Protein (APP)
A role of Amyloid β (Aβ) peptide aggregation and deposition in AD pathogenesis is widely accepted. Toxic peptides are produced by the metabolic processing of Amyloid β Precursor Protein (APP). APP is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. APP proteolysis generates beta amyloid (Aβ), a 37 to 49 amino acid peptide whose amyloid fibrillar form is the primary component of amyloid plaques found in the brains of Alzheimer's disease patients.
Genetic alterations in APP suffice to cause early-onset Familial Alzheimer's disease. Abnormalities induced by aggregated Aβ have been linked to synaptic and neuronal degeneration. This is consistent with the “dying-back” pattern of degeneration that characterizes neurons affected in AD.
Amyloid plaque formation is a central pathological feature of AD. These plaques largely consist of Aβ peptides (4). The Aβ peptides are formed through sequential proteolytic processing of APP made by the β- and γ-secretases (5) (FIG. 1). The γ-secretase is part of the presenilin complex. The aspartyl protease β-site APP cleaving enzyme 1 (BACE1) was originally identified over a decade ago (6, 7). It cleaves APP mostly at a unique site. However, the γ-secretase complex cleaves the resulting carboxy-terminal fragment at several places, especially at positions 40 and 42. The cleavage by this enzyme leads to the formation of amyloid-β1-40 (Aβ1-40) and Aβ1-42 peptides (5). Alternative processing of APP at the α-site prevents the formation of amyloid-β, because the α-site is located within amyloid-β. The neurotoxic potential of the Aβ peptides results from their biochemical properties that favor aggregation into insoluble oligomers and protofibrils. These oligomers and protofibrils accumulate into senile and neuritic plaques. These plaques, along with a reduction of Aβ clearance from the brain, leads to the extracellular accumulation of Aβ. This leads to activation of neurotoxic cascades and ultimately to cytoskeletal changes, neuronal dysfunction and cellular death.
The amyloid cascade hypothesis is based mostly on findings from in vitro and in vivo studies, and is further strengthened by the discovery of genetic mutations associated with early-onset, familial AD. Familial AD is a severe form of the disease, in which massive intra-cerebral amyloidogenesis occurs prematurely as a consequence of mutations all affecting APP metabolism (i.e., mutations in the APP gene in chromosome 21, and in presenilin 1 and 2 (PS-1 and PS-2) genes in chromosomes 14 and 1 respectively). Currently, two proteins are deemed as intimately involved in the clearance of Aβ peptides from the brain: apolipoprotein E (APOE) and the insulin-degrading enzyme (IDE). Disadvantageous genetic polymorphisms (such as the ε4 allele of APOE) and pathological conditions related to abnormal IDE homeostasis (e.g., diabetes mellitus) also favor the amyloidogenic cleavage of APP and/or decrease the Aβ clearance from the brain. This facilitates the accumulation of Aβ in the neural tissues and promote downstream effects of the amyloid cascade (8).
Mutations in APP that are Responsible for Early Onset Familial Alzheimer's Disease
Over thirty coding mutations in the APP gene have been identified (6). Twenty five of these mutations are pathogenic, usually resulting in early onset autosomal dominant Alzheimer's disease (FIG. 2A). Substitutions at or near the β- and γ-proteolytic sites result in over-production of either total amyloid-β or a shift in the Aβ1-40:Aβ1-42 ratio towards formation of the more toxic Aβ1-42 peptide. On the other hand, substitutions within the amyloid-β peptide result in formation of Aβ, which aggregates more easily (9). Mutations in APP are also responsible for the common, late-onset form of Alzheimer's disease.
Mutation that Prevents Alzheimer's Disease
Jonsson et al. (10) searched for low-frequency variants in the APP gene, which significantly reduces the risk of Alzheimer's disease. They studied coding variants in APP in whole-genome sequence data obtained from 1,795 Icelanders. They reported a coding mutation, i.e., an alanine to threonine substitution at position 673 in the APP gene (A673T), which protects against Alzheimer's disease. This mutation is adjacent to the aspartyl protease β-site in APP and is located at position 2 in the amyloid-β peptide. The proximity of A673T mutation to the proteolytic site of BACE1 suggests that this variant might result in impaired BACE1 cleavage of APP.
The A673T mutation reduces by about 40% the formation of amyloidogenic peptides in vitro (10). The strong protective effect of the A673T mutation against Alzheimer's disease provided a proof of principle that reducing the β-cleavage of APP may protect against the disease. Moreover, the A673T mutation also protects against cognitive decline in the elderly without Alzheimer's disease. The carriers of this A673T mutation have a 1.47 times greater chance of reaching the age of 85 without developing AD than non-carriers. Jonsson et al. (10) concluded that the A673T mutation confers a strong protection against Alzheimer's disease. Kero et al. (11) found the A673T variant in one person who died at the age of 104.8 years with little beta-amyloid pathology. This observation supports the concept that this variant protects the brain against β-amyloid pathology and Alzheimer disease.
The present description refers to a number of documents and sequence database entries, the content of which is herein incorporated by reference in their entirety.